1. Field of the Invention:
The present invention relates to an optical disk recording and reading system for recording data on and reading data from an optical disk which has a plurality of sectors, the optical disk recording and reading system having a directory data access mechanism.
2. Prior Art:
There are known optical disk systems which employ non-rewritable optical disks known as write once read many (times) optical disks (WORM optical disks). FIG. 15 of the accompanying drawings shows a data file structure for such a WORM optical disk, the data file structure having a user data area and a directory data area. More specifically, the data file structure includes a user data area 2 which stores user data 1, a directory data area 4 which stores directory data 3 including latest directory data 5, and a virgin area 6. One track corresponds to one circular path on the optical disk.
The directory data 3 contain the file name, address information, size, and attributes of the recorded user data 1. When the optical disk is inserted into an optical disk recording and reading system, the directory data 3 are read by the system. The directory data 3 are updated each time data are subsequently recorded on and read from the optical disk. Actually, since it is impossible to rewrite recorded data on the WORM optical disk, updated latest directory data 5 are newly recorded in a leading sector of the virgin area 6. Therefore, the latest directory data 5 are always recorded in the final sector in the directory data 3 recorded in the directory data area 4, i.e., in the sector at the boundary between the directory data area 4 and the virgin area 6.
When the latest directory data 5 are to be accessed by a conventional WORM optical disk recording and reading system, the directory data area 4 is sequentially searched from its leading end until a first virgin sector is detected, and the directory data 3 recorded immediately in front of the detected virgin sector are read as the latest directory data 5.
One known method of recording on and reading directory data from a WORM optical disk based on the detection of a virgin sector, as described above, is disclosed in Japanese Laid-Open Patent Publication No. 63-14379. An application of such a method is disclosed in Japanese Laid-Open Patent Publication No. 62-6321, for example.
Generally, data can be recorded on an optical disk by burning pits each 1 .mu.m in diameter into the disk surface with a small spot of a laser beam emitted from a semiconductor laser, and recorded data can be read from the optical disk by applying a laser beam to pits and receiving a light beam reflected from and modulated by the pits. The pits are recorded in circular tracks which are radially at spacings of about 1.6 .mu.m, i.e., two adjacent tracks are spaced from each other at an interval or pitch of 1.6 .mu.m. Therefore, optical disks allow storage of large quantities of information at a high density.
One problem with the high-density recording on optical disks is a crosstalk, i.e. when data are being read from one sector on a track, a signal from an adjacent track may also be picked up and mixed into the signal from the sector being read out.
This crosstalk problem will be described in detail with reference to FIGS. 16(a) and 16(b). FIG. 16(a) shows the intensity distribution of a laser beam spot on a recorded surface of an optical disk, and FIG. 16(b) is a radial cross-sectional view of the recorded surface. The recorded surface has pits 7 along tracks 10 which were burned as data by a laser beam. When a laser beam 9 from a semiconductor laser is applied to the recorded surface, a light beam 8 which is modulated by the pits 7 is reflected from the recorded surface.
As can be seen from FIGS. 16(a) and 16(b), the intensity of the laser beam applied to the recorded surface spreads into adjacent tracks, with different light intensities being indicated by the solid-line and broken-line arrows in FIG. 16(b). Even if the laser beam is well converged by an optical lens, it is impossible to focus the laser beam spot solely on the desired track, and light of a small intensity is always applied to the adjacent tracks. Accordingly, some weak noise due to cross-talk has been unavoidable.
In actual optical disk recording and reading systems, it is difficult to focus the laser beam spot well on the desired track from which data are to be read. Due to a focusing error, the intensity distribution of the applied laser beam may become wider than that shown in FIG. 16(a), or the center of the intensity distribution tends to be shifted laterally owing to a slight tracking servo error.
FIGS. 17(a) and 17(b) show the intensity distribution of an applied laser beam and the manner in which light is reflected, respectively, when the laser beam suffers from focusing and tracking errors. When there are focusing and tracking errors, a cross-talk component from an adjacent track is increased. The cross-talk-dependent light beam reflection is however much smaller than the normal light beam 8 reflected from the desired track. Any cross-talk caused by such focusing and tracking errors has been practically harmless insofar as the track containing recorded data pits is read.
When a virgin track is being read, however, cross-talk noise from an adjacent recorded track may cause a problem. More specifically, as shown in FIGS. 18(a) and 18(b), whereas uniform light is reflected from a virgin track, light which has been modulated from pits is reflected from the adjacent recorded track. Because only the modulated light reflected from the disk represents a cross-talk signal from the adjacent track, the data recorded on the adjacent track may be read as if they were recorded on the virgin track.
This erroneous cross-talk signal readout is problematic in that a virgin sector on an optical disk may not fully be confirmed as a virgin sector. The aforesaid system for detecting directory data based on the detection of a virgin sector may not be reliable in operation.